The invention relates to a method for measuring the wall thicknesses of bodies by means of ultrasonic pulses that are produced in the bodies and reflected on the walls, the transit times of the ultrasonic pulses reflected on the walls being measured and the wall thickness determined from the transit times via the sonic speed in the respective body.
It is well known that ultrasonic pulses can be generated in bodies with an ultrasonic transmitter that propagate from one side of the body until they encounter the back wall from which they are reflected. The reflected ultrasonic pulses pass through the body again and are recorded with a receiver test head which converts them into electrical signals. The electrical signals are processed further in order to determine the wall thickness (J. Krautkramer, H. Krautkramer: "Werkstoffprufung mit Ultraschall", 4th edition, Springer Verlag, Berlin/Heidelberg/New York, Pages 191-195). The signals reflected from the back walls are also known as back wall echos.
Only those received ultrasonic signals are evaluated that are present within specified time gates. The beginning of the time gate depends on the time at which the ultrasonic pulses are generated in the body to be tested. The end of the time gate can be set after matching to the approximate wall thickness such that multiple echos do not occur within the time gate. The start and stop times for the time gates are triggered by signals from the transmitter and receiver respectively.
For converting electrical energy into ultrasonic vibrations, piezoelectric and other types of transducers are used. These transducers are also used in the single head mode for receiving the reflected ultrasonic pulses. When piezoelectric transducers are used there is a relatively favourable signal-to-noise ratio with respect to the received ultrasonic pulses. It is therefore possible to specify a base threshold that matches the noise and disturbance voltages and with which the received signals are compared. If the thresholds are exceeded, this information is used for determining start and stop times. In the case of materials with high attenuation of the ultrasonic waves or in the case of great wall thicknesses, depth compensation is provided, i.e. the threshold is reduced and the transit time of the ultrasonic pulses increases.
With ultrasonic transducers operating on the electrodynamic principle, the signal-to-noice ratio for the received ultrasonic pulses is less favourable than with piezoelectric transducers. Furthermore, when electrodynamic ultrasonic transducers are used, the following occurs in particular:
(a) highly dynamic behaviour of the received ultrasonic signals; PA1 (b) high secondary lobes of the received ultrasonic signals (the secondary lobes can differ from the respective main maximum by only approx. -10 dB) PA1 (c) although only transverse waves are excited by the electrodynamic method, longitudinal waves also occur (approx. -12 dB referred to the absolute maximum); and PA1 (d) a relatively high attenuation occurs between the 1st and 2nd back wall echo.